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Cleansing the Waters quest end text.

Of course, the relationship between a demon and any creature they beget or create is entirely unknown to mortals. It could be possible that Aggonar had a direct hand in transforming a mortal mo'arg, Aggonis, into a demon through a blood rite, entirely created him in the shape of a mo'arg felguard (unlikely, as few eternals — let alone mid-ranking demons — have the power to create from scratch) or that Aggonis is a shapeshifter (as are many other demons) and that he simply prefers to assume the shape of a felguard over his true annihilan form (though it is confusing as to why he would take the form of a weaker demon).